This invention relates to a field-effect transistor and, more particularly, to a field-effect transistor for high frequency application and power amplification.
In order to improve the high-frequency properties of a field-effect transistor, for example, a metal oxide semiconductor field-effect transistor (hereinafter referred to as "MOS FET"), it is necessary to reduce gate resistance Rg, source resistance Rs, and gate input capacity Ciss, and to increase mutual conductance gm. To this end, it is the common practice in the manufacture of high-frequency MOS FETs to form the source and drain regions in self-alignment with the gate electrode. In order to form the source and drain regions in self-alignment with the gate electrode, the gate electrode must be made of a compound of silicon with a refractory metal such as molybdenum (Mo), tungsten (W), tantalum (Ta), titanium (Ti), etc. The self-aligning fabrication of the source and drain regions is carried out by implanting an impurity ion in the substrate, with the gate electrode used as a mask, and by annealing at a temperature around 1000.degree. C. in order to activate the implanted ions.
However, where the gate electrode is made of a low-melting point metal such as aluminium, which is widely used as an electrode material, the gate electrode undesirably tends to melt during the heating process. Therefore, in order to form the source and drain regions in a self-aligned manner, it is necessary to fabricate the gate electrode from a pefractory metal.
Also, for the manufacture of a power-amplifying MOS FET device for high power applications, it is common practice to connect a plurality of small-scale FET cells in parallel, so as to elevate the mutual conductance gm and to minimize heat resistance.
FIGS. 1 and 2 illustrate the arrangement of a conventional MOS FET for high frequency and power amplification, wherein the gate-electrode is fabricated from a refractory metal.
FIG. 1 is a plan view of the pattern of the prior art MOS FET. FIG. 2 is a sectional view along line A--A' of FIG. 1.
Referring to FIGS. 1 and 2, reference numeral 40 represents a P.sup.+ -conductivity type silicon substrate; reference numeral 41 denotes a P-conductivity type epitaxial silicon layer; reference numeral 42 shows a silicon oxide layer acting as a gate insulating layer; reference numeral 43 denotes a gate electrode made of a compound of refractory metals-for example, molybdenum and silicon; reference numeral 44 shows an interlaid insulation film; reference numeral 45 indicates a source electrode made of a metal such as aluminium; reference numeral 46 represents a drain electrode made of the same metal as source electrode 45; reference numeral 47 denotes a contact hole through which a drain layer (not shown) is connected to drain electrode 46; reference numeral 48 shows a contact hole through which a source layer (not shown) is connected to source electrode 45; reference numeral 49 indicates a bonding electrode for leading out the gate electrode; reference numeral 50 denotes a contact hole through which gate electrode 43 is connected to bonding electrode 49 for leading out the gate electrode; reference numeral 51 shows a bonding electrode for leading out a drain electrode; and reference numeral 52 represents a bonding electrode for leading out a source electrode.
An MOS FET whose gate electrode is made of a compound of a refractory metal and silicon has the drawback in that the specific resistivity of the gate electrode is about 2 to several hundreds of times higher than that of the gate electrode made of aluminium. An electrode made of only molybdenum or tungsten, both having a relatively low resistivity, tends to react vigorously with water, thus eventually leading to the unreliability of the resultant MOS FET. With the prior art field-effect transistor, therefore, the gate resistance Rg could not be sufficiently reduced, thus imposing limitations on the high-frequency property of the MOS FET.
In a high-power apparatus comprising a plurality of cells of MOS FETs, those cells apart from the bonding electrode for leading out the gate electrode have greater resistance because the gate electrodes of the cells are connected together via a common gate electrode layer. Therefore, the high-frequency properties of the transistor are unsatisfactory.
Further, the manufacture of a low-noise high-frequency field-effect transistor presents difficulties because the gate resistance can not be reduced, and thus, high power output and noise reduction cannot be attained.
As described above, in the prior art field-effect transistor, the gate resistance cannot be reduced, and therefore, a high power output and noise reduction cannot be attained.